Fibromyalgia (FM) and chronic fatigue syndrome (CFS) are two conditions of unknown cause that share many symptoms in common, including painful muscles and joints, fatigue, poor sleep, difficulty thinking and worsening of symptoms during periods of physical or emotional stress. Patients with FM or CFS often have associated symptoms of chronic headaches, temporomandibular disorders, irritable bowel or bladder, and pelvic pain. In addition, patients often describe themselves as anxious or depressed. Our group has proposed that FM, CFS, and other related disorders, could be termed 'stress-associated' syndromes. The response to stress involves secretion of a series of hormones in the hypothalamus of the brain, pituitary gland, and adrenal gland (the HPA axis). These hormones are thought to provide important mind-body links for symptoms including pain and fatigue. This study is designed to understand how the stress hormones ACTH and cortisol are secreted in patients with FM or CFS. We hypothesize that we will find differences in the activity of the HPA axis hormones in FM and CFS patients in the resting state and when the stress response is activated. We will study patients that are free of significant medical illness and substances that can affect stress hormones such as medications for pain, sleep, depression or anxiety and nicotine. Patients will be paired with healthy control subjects of similar age, gender, hormonal status, and body weight. Patients and control subjects will undergo one or more tests of stress response hormones. The first study involves frequent (every 10 min) measurements of ACTH and cortisol in the blood under basal (resting) conditions over 24 hours. After the initial studies, patients undergoing this type of testing will take a medication (metyrapone) that affects the way hormone secretion is controlled to determine whether there is altered activity of the central components of the stress axis which reside in the hypothalamus of the brain. Other studies involve administration of hormones, corticotrophin releasing hormone (CRH) or arginine vasopressin (AVP), to determine the effects on the release of ACTH and cortisol in the blood. Taken together, these tests will provide inferential information about the hypothalamic component of the HPA axis. As a result of these studies, informed development of reasonable approaches to diagnosis and treatment will become more likely.